This invention relates to a transducer element suitable for use in a torque or force sensor and to a transducer assembly incorporating the element.
One approach to contactless sensing of torque in a shaft rotating about its axis is a torque sensor based on magnetoelasticity. A magnetoelastic transducer element is secured to or integral with the shaft, the torque in which is to be measured, and a torque-dependent magnetic field emanated by the transducer element is detected by a sensing device external to the shaft, but not in contact with the shaft, and responsive to the emanated magnetic field. Examples of sensing devices are a Hall effect device, a saturating coil sensor, or various of other magnetic field sensitive devices known in the art. It will be understood that in practice a sensing device may be an assembly of devices. For example, a plurality of sensing devices may be disposed about the axis of the shaft and interconnected to be additive with respect to the torque-dependent field but to cancel in respect of external fields such as the Earth""s magnetic field.
Magnetoelastic transducer elements previously proposed form a ring or annulus which is circumferentially magnetised. The field forms a closed loop normally contained within the element. One form of transducer element is a separate ring of magnetoelastic material attached to the shaft such as disclosed in U.S. Pat. Nos. 5,351,555, 5,465,627 and 5,520,059, all to Garshelis and assigned to Magnetoelastic Devices, Inc. In the ring transducer elements, the ring supports a circumferential magnetic field which is confined within the ring, that is no field is detectable externally in the absence of torque. When torque in the shaft is transmitted to the magnetoelastic ring, an external magnetic field is emanated and is detected by a sensor arrangement.
A different approach to providing a circumferentially magnetised magnetoelastic sensor is disclosed in International Patent Application PCT/GB99/00736 (published on 4th Nov., 1999 under the number WO99/56099) in which the transducer element is an integral portion of the shaft whose torque is to be measured. This avoids problems in securing a separate ring properly to the shaft. An integral transducer element approach is also disclosed in published International Patent Applications WO99/21150 and WO99/21151.
Magnetoelasticity is a phenomenon which, as yet, is apparently still not fully understood and explained. It is, therefore, generally desirable to find other forms of magnetisation that might be employed in transducer elements, particularly suitable for torque sensing.
A disadvantage of torque transducer elements that are circumferentially magnetised is that it is difficult to calibrate the sensor system with respect to short term field variations with temperature or longer term changes of the magnetic field. A transducer element which produces no reliably detectable field under no torque presents a calibration problem.
Reliability and longer term stability are also enhanced in a preferred torque transducer system described in PCT/GB99/00736 (WO99/56099). The shaft is directly magnetised in three or more regions along the axis. Taking the case of three regions, an inner region is circumferentially magnetised with one polarity and it is flanked by respective outer regions magnetised with the opposite polarity of circumferential magnetisation. The inner region provides a transducer element, the two adjacent outer regions acting as guard and keeper regions.
According to a first aspect of the present invention, one or more magnetic transducer elements are provided integrally in a shaft of magnetisable material but using longitudinal magnetisation, that is a magnetisation that lies in an axial direction in contrast to circumferential magnetisation.
More particularly, the provision of three or more longitudinally magnetised regions having an inner region flanked by two regions of opposite polarity to the inner region enables the inner region to be used as the transducer element while the two flanking regions act as guard or keeper regions for it. More than three regions of alternating polarity may be provided with inner regions acting as transducer elements and as a keeper and guard region for an adjacent inner region. The provision of additional regions as keeper or guard regions help maintain the magnetisation of the transducer element region and isolate it from other fields induced in the shaft, especially where the transducer is used in the presence of strong magnetic fields. These measures to enhance the stability of a longitudinally-magnetised transducer and mitigate the effect of other fields in a shaft on the element need not necessarily employ longitudinal magnetisation for guard region purposes. Non-transducer element guard regions may be circumferentially magnetised with a view to providing a stable magnetic environment within which the transducer element operates.
Present investigations have indicated that the invention can be practised irrespective of whether the material exhibits magnetoelasticity though many materials will do so in any event. It is a feature of the longitudinal magnetisation proposed that a magnetised region will exhibit a fringing field external to the shaft whose direction is a function of torque and which can be used as a reference for calibration purposes. The invention may be practised with a magnetisation that is essentially confined to an annular surface zone of the shaft. The longitudinal magnetisation disclosed herein and discussed below is detected by emanating a torque-dependent field that has a tangential or circumferentially-directed component. This form of magnetisation may be referred to as circumferential sensing, longitudinal magnetisation. The axial or in-line component of the external field, which exists even at zero torque, may be utilised as a reference.
The present invention also includes the concept of measuring the bending force or the shear force in an elongate member subject to a bending or shearing moment.
For convenience all such elongate members, subject to torque, bending and/or shear forces, whether intended for rotation or not, will be referred to as xe2x80x9cshaftsxe2x80x9d. The invention will be mainly discussed and described in relation to a shaft rotatable about a longitudinal axis to transmit torque applied to a driven end of the shaft to a load coupled to the other end. However, it will be understood that torque measurement can be required in some circumstances where the load end of the shaft is effectively fixed and forces inducing torque are applied at the other end.
The invention will also be discussed and described in relation to a ferromagnetic shaft of solid circular cross-section. It will be understood from what follows that the shaft may be of other cross-sectional shape as regards its circumference and that non-solid sections may be usable in the practice of the invention. For example, a hollow shaft may be magnetised in the manner to be described provided it has sufficient wall thickness to sustain the desired longitudinal magnetisation.
Aspects and features of the present invention for which protection is sought are set out in the accompanying claims.